It is often necessary, or at least desirable, that cardiac output be monitored or measured. While apparatus and methods have been heretofore suggested and/or utilized to accomplish this end, such apparatus and/or methods have not been completely satisfactory and have required, for example, mixing of calibrating solutions, withdrawal of blood and/or special preparation of the indicator (such as cooling), which has limited the usefulness of such methods and/or resulting apparatus.
In addition, while dilute saline has heretofore been utilized for the measurement of cardiac output (see, for example, Stewart, G. N. "Researches on the Circulation and on the Influences Which Affect It" Journ. Physiol. 22:159-183 (1897) and Stewart, "The Output of the Heart in Dogs" Amer. Journ. Physiol. 57:27-50 (1921)), it is now seldom used even though use of such an indicator has attractive features including low cost, low toxicity and enables the use of a simple and inexpensive detector, namely a conductivity cell.